Since LED devices using InGaN as a nitride semiconductor compound were developed in the mid-1990's, white LED devices have opened a new era of semiconductor illumination. White LED devices are advantageous over incandescent bulbs (e.g., popular 60 W type bulbs) in terms of long service life, small size and driving at low voltage. Based on these advantages, white LED devices have recently been applied to a wide variety of fields, including household fluorescent lamps and full-color displays (LCD backlights).
Conventional methods for fabricating white LED devices utilize three-color LEDs (i.e., red, green and blue LEDs). However, the conventional methods have the problems of high fabrication costs and complicated driving circuits, which increases the size of the final devices. In addition, since the three LEDs have different temperature characteristics, the optical properties and reliability of the final devices may be adversely affected.
Under these circumstances, a method for producing white light by combining a yellow phosphor or a phosphor mixture of a green phosphor with a red phosphor with a 450 nm InGaN-based blue LED has been developed. The principle of white light emission is as follows. First, a portion of blue light emitted from a blue LED excites the phosphor(s) to generate yellowish green fluorescence. Then, the blue light and yellowish green light are combined together to emit white light. FIG. 1 shows a cross-sectional view of a conventional white LED device using a yellow YAG:Ce phosphor powder. As shown in FIG. 1, the white LED device is fabricated by applying the yellow YAG:Ce phosphor powder to an LED. The white LED device has a low color rendering index (CRI), making it difficult to emit white light approximating sunlight, but the device advantageously has a relatively high luminescent efficiency.
For the purpose of increasing the color rendering index of white LED devices, a number of efforts have recently been made to develop white LED devices capable of emitting white light approximating sunlight by using a combination of a UV/violet LED and a three-color (blue, green and red) complex phosphor, instead of a combination of a blue LED and a yellow phosphor. FIG. 2 is a cross-sectional view schematically showing a white LED device using a three-color complex phosphor powder. With reference to FIG. 2, since the three-color complex phosphor is excited by light at 420 nm or shorter from a violet or UV LED to emit white color, the color rendering index of the white LED device is markedly increased. In addition, since the mixing ratio between the phosphors is controlled, there is the advantage that the chromaticity of the white LED device can be easily controlled.
A phosphor excited by a blue or UV/violet LED is essentially required for the fabrication of a white LED device. However, since phosphors used hitherto are powder type, they scatter or absorb large proportions of light from excitation light sources and light generated from the phosphors excited by the sources, causing the problem of low luminance. Further, the phosphor powders must be used in a slurry form in order to apply the phosphors to LEDs. However, phosphor powders in a slurry form disadvantageously have poor physical and chemical homogeneity. Further, some of the phosphors participate in light emission, but some of the phosphors do not due to scattering and screen effects. Moreover, since the amount of the phosphors used is large, considerable production costs are incurred.
On the other band, when the phosphors are produced into a thin film, physical and chemical homogeneity is improved, adhesion to substrates is superior, gas evolution is minimized, and amount of the phosphors used is reduced. However, since a major portion of light generated from the phosphor thin films is not escaped from the thin films due to total internal reflection or light piping effect, the luminescent efficiency and luminance of the devices are fatally deteriorated. Furthermore, in the case that the crystallinity and transparency of the phosphor thin films are poor, light is additionally lost inside the phosphor thin films other than light loss due to total internal reflection or light piping effect, and thus the luminescent efficiency of the devices is further lowered. For these problems, phosphors in a thin film form have not been used to fabricate LED devices until now.